5A BS 1363 plug?

[stillp]

I was replying to your assertion that the performance of a given type of cable at a given temperature has nothing to do with the current it is deemed capable of carrying.

That was not my assertion. I stated that it was not directly related.
There are many flexible cables that are suitable for use at conductor temperatures in excess of 70C. Not common in a domestic situation perhaps, but common in industry. It is also possible, in many situations, to limit the flexing and other stresses on the cable, or apply forced cooling. Again not practicable for domestic appliances, but I was not restricting my comments to domestic appliances or to the scope of BS 7671.

 

[plugwash]

As an asside I do wonder how the nominal current ratings of flexible cables were determined, they certainly don't seem to fit the expected curve for thermally determined ratings .

Heat production is proportional to the square of current and to resistance. Resistance is is inversely proportional to CSA. Heat dissipation (at a given temperature) is proportional to circumference which is proportional to the square root of CSA. For a cable to be in thermal equalibrium heat production needs to be equal to heat dissipation.

Given all of that we would expect doubling the CSA to halve the resistance and increase the permisable heat production by sqrt(2). That means that the permisable current will increase by 2^(3/4)

Edit: fixed some missing text.

 

[plugwash]

They have been made aware - the manufacturer has stamped the rating of his product on the bearing face of the plug, as per BS 1363. If that says '6A' then the consumer has been made aware that it is unsuitable for any load greater than 6A.

mmm, a difficult case.

On the one hand products with regular (not hot condition) IEC inlets and greater than 5A power draw at 230V are uncommon.

On the other hand normal people are generally ignorant of the fact that IEC cords come in multiple different current ratings. So they wouldn't even think to look for a rating stamped on the connector and if they do they may well look at the wrong one. So if there are any such devices around in an environment that is not very strictly controlled there is a real chance of the power cord getting swapped for an unsuitable one.

On the third hand going back to my asside above I believe that the nominal rating of 0.75mm flex is artificially low and so the risks from the above scenario are much lower than they would first appear. IIRC on a previous similar threat someone dug up an EU standard claming that 0.75mm flex could be used for 10A under certain scenarios (and didn't explain why only in those scenarios).

FWIW I also have a load of IEC cords with 0.75mm flex and unfused schuko plugs hanging around.

 

[JohnW2]

As an asside I do wonder how the nominal current ratings of flexible cables were determined, they certainly don't seem to fit the expected curve for thermally determined ratings .

True, but I think the same is probably also true of the CCCs we work with for most typs of cable.

Heat production is proportional to the square of current and to resistance. Resistance is is inversely proportional to CSA. Heat dissipation (at a given temperature) is proportional to circumference which is proportional to the . For a cable to be in thermal equalibrium heat production needs to be equal to heat dissipation. ... Given all of that we would expect doubling the CSA to halve the resistance and increase the permisable heat production by sqrt(2). That means that the permisable current will increase by 2^(3/4)

I think that, in practice, it's probably a fair bit more complicated than that. The sort of calculations you are postulating would presumably be appropriate if one were thinking about a bare conductor, with the dissipation of heat all being from the surface of that conductor. For a real-world multicore insulated and sheathed cable, dissipation into the surroundings will be from the surface of the outer sheath. That sheath will have a much larger surface area than the conductor, with that surface area having a 'far-from-simple relationship to the CSA (or diameter,or circumference) of the conductor. Whilst the 'heat production'part of the equation is pretty straightforward, the heat loss/dissipation part is therefore much less straightforward. Although it's a pure guess (well, intuition) unless/until I do some sums, I would suspect that, with real-world cables, even with simple geometry, the surface area of the outer sheath probably increases proportionately a fair bit less than does the surface area of conductor as conductor CSA increases.

One imagines that the current ratings/CCCs of cables we work with derive from empirical data, and hence are probably roughly 'correct'.

The related question I posed was why (at least as far as BS7671 is concerned), the CCCs of flexible cables are, at least for smaller CSAs, much lower than those of, say, T+E of the same CSA. It was suggested to me thatthe reason is the mechanical stresses to which flexible cables might be subjected.

Kind Regards, John

 

[plugwash]

True, but I think the same is probably also true of the CCCs we work with for most typs of cable.

I think that, in practice, it's probably a fair bit more complicated than that. The sort of calculations you are postulating would presumably be appropriate if one were thinking about a bare conductor, with the dissipation of heat all being from the surface of that conductor. For a real-world multicore insulated and sheathed cable, dissipation into the surroundings will be from the surface of the outer sheath. That sheath will have a much larger surface area than the conductor, with that surface area having a 'far-from-simple relationship to the CSA (or diameter,or circumference) of the conductor

It would be interesting to run a curve fit to try and fit an I=A^n relationship to the BS7671 current carrying capacities and see what power it came out with and how closely it fitted.

Some quick checks on spot values seems to indicate that the BS7671 values for T&E are more sublinear than I expected. Probablly because the diameter of the insulated conductor increases slower than the diameter of the conductor itself.

If anything i'd expect the progression of current rating with cable size to be even more sublinear with small cable sizes because the insulation thickness is a much greater component of overall diameter.

And yet for ratings of small flex the progression is super-linear. 0.5mm flex is conventually rated at 3A, 0.75mm (0.5x1.5) at 6A (3x2). 1mm (0.75x1.33) at 10A (6x1.66) 1.25mm (1.0x1.25) at 13A (10x1.3).

 

[JohnW2]

It would be interesting to run a curve fit to try and fit an I=A^n relationship to the BS7671 current carrying capacities and see what power it came out with and how closely it fitted.

Yes,it would be interesting - maybe I'll try!

Some quick checks on spot values seems to indicate that the BS7671 values for T&E are more sublinear than I expected. Probablly because the diameter of the insulated conductor increases slower than the diameter of the conductor itself. ... If anything i'd expect the progression of current rating with cable size to be even more sublinear with small cable sizes because the insulation thickness is a much greater component of overall diameter.

Indeed - that's essentially what I suggested would probably be the case.

And yet for ratings of small flex the progression is super-linear. 0.5mm flex is conventually rated at 3A, 0.75mm (0.5x1.5) at 6A (3x2). 1mm (0.75x1.33) at 10A (6x1.66) 1.25mm (1.0x1.25) at 13A (10x1.3).

Quite so - it does seem that the figures for flex are rather odd. As CSA increases, the flex CCCs get closer to the corresponding CCC for, say, T+E - which might support the idea that the CCCs of the smaller flexes have been 'derated' because of the possible mechanical stresses suffered by flex (which are probably more likely to be an issue with the smaller cables) - but that is pure speculation!

Kind Regards, John

 

[ban-all-sheds]

I was replying to your assertion that the performance of a given type of cable at a given temperature has nothing to do with the current it is deemed capable of carrying.

That was not my assertion. I stated that it was not directly related.

Oh OK.

What is it directly related to, then?

Why isn't the tabulated capacity of 1.5mm² flex 50A?

 

[stillp]

What is it directly related to, then?

Why isn't the tabulated capacity of 1.5mm² flex 50A?

If there is a direct relationship, it will be to some function of the surrounding environment, CSA, resistivity, and probably a few other things.

I don't know. Tabulated by whom?

 

[ban-all-sheds]

If there is a direct relationship, it will be to some function of the surrounding environment,

Such as?

CSA

Well, yes, the CCC of a cable does depend on it's CSA, but why?

resistivity

OK - the resistivity of copper is 1.68×10^−8 Ω⋅m, so why does that matter?

and probably a few other things

Any suggestions?

I don't know. Tabulated by whom?

The IET, for example, in Table 4D5

Why are none of those values 50A?

 

[stillp]

Dunno. Perhaps because the standard methods used in installations do not allow common forms of 1,5mm2 flex to carry 50A.


Why have you asked a question about flex and posted a table that appears to relate to T & E?

 

[ban-all-sheds]

Dunno.

Like to hazard a guess?

Advance any theories?

Suggest any possibilities?

Perhaps because the standard methods used in installations do not allow common forms of 1,5mm2 flex to carry 50A.

Why not?

Why have you asked a question about flex and posted a table that appears to relate to T & E?

Mainly because I knew where to find the table without searching. The principle is the same, surely? They are both PVC insulated copper cables. If you can explain why 1.5mm² T+E isn't rated at 50A then wouldn't that be relevant to 1.5mm² flex?

But if that is a stumbling block, here you go:

Why aren't those values 50A?

 

[JohnW2]

<images wider than my screen ... twice>

Kind Regards, John

 

[ban-all-sheds]

Sorry, John, but there comes a point where you cannot reasonably expect the rest of the world to adapt to your limited display facilities.

And that point is when you think people should shrink images to the extent that the text in them becomes smaller than the default font on the site. The second one, yes, I could have made it smaller, but the column heading text in the first one really is already as small as it ought to get.

 

[stillp]

I can only see an icon.

 

[ban-all-sheds]

I can only see an icon.

For the second one?

http://s1.postimg.org/ii0g0ci1r/screenshot_312.jpg